The present invention relates to a power supply control circuit that is externally attached to a stabilized direct current (DC) power supply. The stabilized DC power supply has a pair of output terminals and a pair of remote sensing terminals. The power supply control circuit performs constant current control for the stabilized DC power supply. The present invention also relates to a power supply device that has the power supply control circuit and the stabilized DC power supply explained above.
As this kind of power supply control circuits, a power supply control circuit disclosed in Japanese Patent Number 3683548 has been known. This power supply control circuit controls a stabilized DC power supply in which a positive output terminal and a positive remote sensing terminal are connected. A load is connected between the positive output terminal and a negative output terminal of the stabilized DC power supply. Further, the power supply control circuit explained above has a resistor (a resistor for current detection) and an operational amplifier. The resistor is provided on a side of the negative output terminal and is connected to the load in series. The operational amplifier corresponds to a variable voltage source. Further, the above resistor has one terminal, which is located at a side of the load, connected to an inversion input terminal of the operational amplifier. An external control voltage (a first external control voltage) is input to a non-inversion input terminal of the operational amplifier (specifically, the external control voltage is input by being divided). A voltage of the resistor's other terminal, which is located at a side of the negative output terminal, is a reference voltage for the external control voltage. Further, an output (a control voltage) of the operational amplifier is connected to the negative remote sensing terminal.
In the power supply control circuit explained above, the operational amplifier is operated by a negative voltage and a positive voltage. Specifically, the negative voltage is output from a negative electrode terminal of an auxiliary power supply in which a positive electrode terminal is connected to the negative output terminal of the stabilized DC power supply. The positive voltage is output from the positive output terminal of the stabilized DC power supply. Further, the operational amplifier compares a voltage that is generated at the resistor when a current flowing in the load flows in the resistor (the resistor for current detection) with the first external control voltage that is divided so as to adjust the control voltage that is output to the negative remote sensing terminal. As a result, because constant current control by which the stabilized DC power supply performs a constant current operation is conducted, the current that flows in the load is controlled to be constant. According to the configuration explained above, the power supply control circuit can freely change a set current of the constant current control by changing the first external control voltage.
Further, JP 3683548 discloses another power supply control circuit in addition to the circuit explained above in which the stabilized DC power supply performs the constant current operation. Specifically, the other power supply control circuit has a circuit in which the stabilized DC power supply performs a constant voltage operation. The circuit in which the constant voltage operation is performed is configured with an operational amplifier, an input resistor and a feedback resistor. Specifically, the input resistor is connected between an inversion input terminal of the operational amplifier and the negative terminal of the load. The feedback resistor is connected between the inversion input terminal of the operational amplifier and an output terminal. Thus, the circuit explained above is configured as a non-inversion amplifier circuit that operates based on a voltage of the negative terminal of the load as a reference. Further, the operational amplifier of the non-inversion amplifier circuit also operates by the negative voltage and the positive voltage. The negative voltage is output from the negative electrode terminal of the auxiliary power supply in which the positive electrode terminal is connected to the negative output terminal of the stabilized DC power supply. The positive voltage is output from the positive output terminal of the stabilized DC power supply. As a result, the operational amplifier of the non-inversion amplifier circuit amplifies and outputs a second external control voltage that is divided by the resistor. The second external voltage is input based on the voltage of the negative terminal of the load as the reference.
In the power supply control circuit explained above, the output of the operational amplifier that configures the circuit in which the constant current operation is performed and the output of the operational amplifier that configures the circuit in which the constant voltage operation is performed are combined via a diode and makes the combined output as the control voltage that is output to the negative remote sensing terminal. As a result, because the output current and the output voltage of the stabilized DC power supply can be controlled by the first and second external control voltages, the stabilized DC power supply can be operated in a desired output state of a constant voltage and a constant current.
The power supply control circuit described above, however, still has some problems to be solved. Specifically, because the power supply control circuit explained above needs not only the voltage that is output from the auxiliary power supply but also the voltage that is output from the positive output terminal of the stabilized DC power supply as an operating voltage, problems such as the operation being unstable in a low voltage range of the voltage that is output from the stabilized DC power supply, may exist in the power supply control circuit.